1. Field of the Invention
The present invention relates to the structure of a semiconductor device in which a thin-film transistor (hereinafter abbreviated as “TFT”) is disposed on a substrate having an insulating surface thereof. In this specification, the term semiconductor device designates all the devices that can function using a semiconductor, and all of electro-optical devices, semiconductor circuits and electronic devices are contained in the category of the semiconductor.
2. Description of the Related Art
In recent years, a technique by which a TFT is formed on a substrate having an insulating surface to constitute an electric circuit has been rapidly developed. In the present state, there are many cases in which the TFT is used as a switching element of the liquid-crystal display device (liquid-crystal panel). The active layer which is the most important portion of the TFT is formed of a semiconductor thin film. Up to now, an amorphous silicon film has been mostly employed as the semiconductor thin film. However, a demand for a TFT higher in operation speed has been increased as a result of which a crystalline silicon film (polysilicon film) has been main stream.
As a method of manufacturing a conventional crystalline silicon film, there has been known a solid-phase growing method (thermal annealing method) or a laser annealing method.
The solid-phase growing method, requiring a temperature as high as 600° C. or more, needs to use a quartz substrate high in heat resistance and expensive, and it has been difficult to use an inexpensive glass substrate. Also, the solid-phase growing method is liable to cause irregularity in crystallization, thereby making it difficult to obtain required crystallinity over a wide area.
On the other hand, the laser annealing method is suitable for reducing the costs of the integrated circuit of the TFT because crystallization can be conducted at a process temperature of 550° C. or less without giving a thermal damage to the substrate so that an inexpensive glass substrate can be used. Also, the conventional laser annealing method is difficult to obtain an excellent annealing effect over a large area although high crystallinity can be partially obtained because an area to which a laser beam is applied is small. In particular, the application of a laser beam under the condition where the excellent crystallinity is obtained is liable to be unstable.
As means for solving the above problem, there is a method of promoting crystallization by using a predetermined catalytic element according to the invention made by the present inventors.
This method is a method in which the catalytic element represented by nickel is introduced into an amorphous silicon film, and thereafter a heat treatment is conducted to obtain a crystalline silicone film. In this method, an excellent annealing effect can be obtained over a large area by conducting the heat treatment at a temperature of about 600° C. or less at which the glass substrate can be employed.
Although this method is shorter in processing period than the above-mentioned solid-phase growing method, it still requires the processing period of several hours. For that reason, this method is inferior in mass production to the above-mentioned laser annealing method which is extremely short in processing period.